It must be recalled that in addition to the methanol synthesis, which produces selectively this alcohol, two types of procesess have been mainly proposed for the preparation of alcohols from a synthesis gas of CO+H.sub.2 : the Fischer and Tropsch synthesis and the isobutylic synthesis.
As a general rule, the processes of the Fischer and Tropsch type are not very selective; there is obtained a mixture of olefinic, paraffinic and oxygenated products over a very wide range of molecular weights. Moreover, the productivity of the catalyst is very low, generally lower than 5 kg of products per ton of catalyst and per hour.
As concerns the isobutylic synthesis, performed in Europe from 1935 to 1945, it derives from the methanol synthesis by the use of the same catalyst (zinc chromite) modified by the addition of an alkaline salt, but it is operated under higher pressures and temperatures (respectively 300 to 400 bars, 380.degree. to 450.degree. C.); a representative composition by weight of the products obtained is as follows: methanol (50%), isobutanol (20-40%), n-propanol and higher alcohols (complement to 100%); the higher alcohols consist of primary and secondary (50-50%) non linear alcohols.
Processes of the prior art are used to produce, in at least two reactors in series, an alcohol (usually methanol) or a mixture of alcohols, from a synthesis gas (French Pat. No. 1,046,822, Patent of the Federal Republic of Germany Pat. No. 1,257,762 and U.S. Pat. No. 1,959,219); but these processes always comprise a recycling to the first reactor of at least one portion of the residual gases from one or more reactors, this recycling being performed in order to increase the yield of alcohol(s) in the first reactor but having the disadvantage of resulting in further investment expenses. The process of the invention, on the contrary, does not require this costly recycling since it has precisely for objet to avoid the production of a too large amount of linear and saturated higher homologs of methanol in the first reaction zone in view of the fact that the desired final mixture of methanol with homolog alcohols should have a content of homolog alcohols not exceeding 26% by weight. Briefly stated, this invention proposes a combination of processes where, to a synthesis of methanol is coupled a synthesis of homolog alcohols.